Variable delay control circuit for motor-driven signal seeking receivers



Feb. 5, 1952 R. A. GULL 2,584,578 VARIABLE DELAY CONTROL CIRCUIT FOR MOTOR-DRIVEN SIGNAL SEEKING RECEIVERS Filed Nov. 14. 1947 25 22 J-/z'sll- Yjfqg' GyaZZ Patented Feb. 5, 1952 VARIABLE DELAY CONTROL CIRCUIT FOR MOTOR-DRIVEN SIGNAL SEEKING RE- CEIVERS Roy A. Gull, Bualo, N. Y., assignor, by mesne assignments, to Sylvania Electric Products Inc., New York, N. Y., a corporation of Massachusetts Application November 14, 1947, Serial No. 785,994

3 Claims.

`This invention relates to an improved control circuit for signal seeking receivers. In signal seeking receivers, so-called, the tune: is driven by a motor and is-arranged to scan the spectrum of the receiver repeatedly and control apparatus is provided whereby when a signal of predetermined strength is tuned in, the motor driving the tuner is disconnected, permitting the receiver to remain tuned to the incoming signal. In such receivers usually a vacuum tube is used as the control element, the incoming signal be- -ing rectified and applied to the grid of the control tube in a manner to reduce or cut oi the 'plate current of the control tube, thereby permitting a relay to open the motor circuit. In normal signal seeking receivers using a hard control tube, with a very selective piezo-electric crystal or when using a high scanning speed, 4the control tube may be cut oil for only such a veryshori; length of time that the relay does not have time to release and the receiver may tune through a signal instead of stopping as it should. To overcome this diiculty in the, past, `it has been necessary to use a relatively low scanning speed. This, in turn requires an excessive length of time for the receiver to scan vfrom one end of the spectrum to the other. A1- ternatively, less selective crystals have been used giving a broader frequency response. Neither of .these `expedients is satisfactory from the scanning speed standpoint. Y

It is an object of this` invention, to providek a `system vwhich permits the scanning speed of a signal seeking receiver to be greatly increased or,

alternatively, permits the use of a fixed scanning beconsiderably increased without the danger of r`the receiver passing over an incoming signal instead of stopping.

It is still a further object of this invention to `provide .an improved control circuit in which vairous, adjustments may be m-ade to control the action of the crystal in stopping the receiver.

It is a further object of this invention to proyide a system of the class described in whichthe plate current of the control tube remains at a value suiiiciently low for a time long enough to permit the motor control relay to drop out, even though the length of time a signal is tuned in is much less than this value.

Still other objects and advantages of my in- Jention will be apparent from the specification.

The features of novelty which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its fundamental principles, and as to its particular embodiments, will best be understood by reference to the specification and the accompanying drawings in which:

Figure 1 is a circuit diagram of a signal seeking receiver embodying my invention,

Figure 2 is a set of curves explaining the operation of the circuit according to my invention.

Turning now to Figure 1, I 0 designates any suitable. antenna supplying incoming signals to radio frequency amplifier, oscillator and converter unit ,II which may comprise tuned circuits formed by lnductance I2 and condenser I3 and inductance I4 and condenser I5. Still other tuned circuits may be employed but are omitted for the sake of simplicity. The tuned circuits may be tuned by driving motor I9 which may for example, move permeability tuning cores into inductances I2 and I4, although variable condensers may be used if desired.

The output` from the-amplifier, oscillator an converter unit I I is supplied to the intermediate frequency amplifier I6 and thence,v through de tector andaudio amplifier unit I1 to loud speaker I8. To provide the desired control, a portion of the output from intermediate frequency amplifier I6 is supplied to limiterZII. The limiter 20 may be in the form of a Vacuum tube amplifier arranged to deliver an output which never exceeds a predetermined fixed value no matter how great the input. The output of the limiter may then be supplied to the crystal bridge circuit comprising transformer 2|, the primary coil being shunted by trimmer condenser 22 and the secondary ,coil being shunted by condensers 23 and .24 in series with their common terminal connected to ground. The arrows in Figure 1 indi cate permeability cores, that is, cores' formed of iron dust mixed with a binder.

One side of the secondary of the bridge transformer 2l supplies one terminal of a piezo-electric crystal 25, the other side of which is connected through adjustable condenser 26 to the opposite terminal of the secondary of the bridge transformer 2l. The crystal 25 is made resonant to the center of the intermediate frequency band and preferably has a band width of only about fr; or less of that of the preceding circuits; for example, if the intermediate frequency ampliiier passes a band width of 5,000 cycles, the crystal will pass a band width of only `500 cycles or less.

This particular feature is not, per se, a part of this invention and is included only for the p urpose of completing the disclosure of the present invention. The feature referredk to is described and claimed in a co-pending application of Verlis H. Wiley, Serial No. 554,470, now United States Letters Patent No. 2,491,942, grantedv December 20, 1949.

The common terminal of crystal and condenser 28 is connected to the rectifier tube. In the present instance, I have shown the rectifier and control tube as a twin triode with the rectifier section made up of the rst half with grid and plate connected together. The'tube 21 may comprise cathode 2101, control electrode 21g1, and anode 21a1 in the iirst'. triode section and cathode 21c2, control electrode 2192 and anode 21a2 in the second triode section, all within vthe same envelope. HoweVen'separate tubes may be used if desired.

Cathode 21c1 may be connected to ground through resistance 3l bypassed by condenser 32, control electrode 21m may be connected to ground through resistance 28 and is also connected to anode 21a1 and through resistance 29 to control electrode 2192. A connection to ground between resistance 29 and control electrode 2192 is provided by condenser 30. The cathode 2102 may be connected directly to ground.

The anode 21a2 is connected through the'energizing winding oi relay 36 to the sourceiplus B, the negative side of which is usually grounded. Cathode 2101 may be connected through resistance 33 to plus B. The relay 34 controls armature 35 which engages fixed contact 36. Armature 35 may be grounded and fixed contact 36 may control the motor circuit through apparatus omitted for the sake of simplicity, in suchca manner that when relay 34 opens, the motor 'circuit is opened.

Generally a sensitive relay of the type of relay 34 has not suflicient current carrying capacity to be placed in the motor circuit directly and usuallyoperates through relays 'capable of carry ing heavier current and it will be understoodthat from contact 3B, there Vmay bea connection through an additional relay which is connected in the motor circuit in such amanner'asf to interrupt the now of power to the motor circuit when relay 34 opens.

The operation of this system will now be described.

When during the normal scanning process, a signal is tuned in, it appears in the loud speaker i8, but a portion of it is passed through limiter 20, crystal bridge transformer 2| and through the crystal tuning circuit or narrow band selector and appears at the grid 2191 of tube 21. It is there rectified and the negative voltage resulting therefrom is applied to grid '2192 through resistance 29. This negative voltage applied tothe grid 21g2 charges condenser 30. When the signal is no longer delivered by the output of the crystal selector through continuation of the scanning process, the condenser 30 tends to maintain the negative charge 0n the grid 21g2 until the charge on condenser 30 leaks oi to ground through resistances 29 and 28. By controlling the discharge time of the condenser 30, the length of time for which the tube 21 is cut off may be greatly increased to allow plenty of time for the sensitive relay 34 to open even with crystals of narrow band width and high scanning speeds.

Suppose that the receiver scans atV the rate of 400 cycles per millisecond and that the maximum crystal output voltage is 100 volts, and that the crystal has a band width of 100 cycles at one-half output or volts on the selectivity curve. Let

:us further suppose that 40 volts of noise delay bias is introduced by the resistance 3 I, 33 voltage divider network and that 10 volts are necessary to cut Voff the control tube, and condenser 30 is omitted. With such a combination, the control tube will be cut off for .250 millisecond as the receiver` scans through a signal. This condition is indicated by curve A of Figure 2. This time is far tooshort to allow most commercial sensitive relays to drop out.

Next, consider what takespiace with condenser 30 in place as shown in Figure l. Let us suppose that resistance 29 is 470,000 ohms and that'resistance 28 is 15 megohms vand condenser 30 is 265 micro-micro-farads. Now the condition existing as the receiver tunes in a signalis shown graphicallyby curve B of Figure 2. In this case, the tube is held cut off-for excess 024.0 milliseconds, orv over 16 times as long as before. This cutoff time can be regulated ory controlled through suitableA choice of the values of resistance 23, resistance 29, and condenser 30. Thus the control tube can be held cut off for a period or time entirely suicient to assure satisfactory sensitive relay operation. Y v

In the condition just described we havenssumed a not unnecessarily sharp crystal with; a. given rate of scan but it will be apparent that the same principles apply when a much 4sharper crystal is used and that although the timewould be even less with a sharper crystal than. .250 millisecond, by application of the principles here'- in disclosed, the timeduring which the control tube is held cut off may be made. entirely sufcient to permit and assure satisfactory `operation 'of the relays.

In the specication I have explained the principles of my invention and the best mode in which I have contemplated applying those principlesso as to distinguish my invention from other inventions, and I have particularly pointed outi'and distinctly claimed the part, improvement', or combination which I claimV as my invention or discovery.

While I have'shown and described certain preferred embodiments of my inventionyit Willrbe understood that modications and changesimay be made without departing' from the spirit and scope thereof, as Will be clear to'those skilled in the art. Y

Icla'im: y

1. In a signal-seeking receiver, in combination, tuning means for eifecting tuning of said receiver continuously with a scanning action over apredetermined range of wave signal frequencies, means for deriving from received wave `signals having an 'intensity exceeding a predetermined value a control potential, the rapidity of said tuning causing said control potential to have a cycle `of amplitude change occurring'during-l a predetermined interval, means including a control circuit responsive to the amplitude of said derived potential for controlling said tuning means to terminate the scanning action thereof, said last-mentioned means including a relay which effects said control of said tuning means consequent upon a change from a rst to a second operating condition of said relay but which requires a minimum interval of change of relay excitation longer than said predetermined interval to eiect said change from said iirst to said second operating condition thereof, and an electrical energy-storage element included with resistors in a charge time-constant circuit and a discharge time-constant circuit in said control circuit for receiving and storing energy supplied to said element by said control potential, the time constant of said charge circuit being substantially equal to said predetermined interval and the time constant of said discharge circuit, being long in comparison to said charge time constant thereby to prolong the amplitude of said control potential over a time interval longer than said predetermined interval.

2. In a signal-seeking receiver, in combination, tuning means for effecting tuning of said receiver continuously With a scanning action over a predetermined range of wave signal frequencies, means for deriving from received Wave signals having an intensity exceeding a predetermined I value a control potential, the rapidity of said tuning causing said control potential to have a cycle of amplitude change occurring during an interval of the order of l millisecond, means including a control circuit responsive to the amplitude of said derived potential for controlling said tuning means to terminate the scanning action thereof, said last-mentioned means including a relay which eiTects said control of said tuning means consequent upon a change from a first to a second operating condition of said relay but which requires a minimum interval of change of relay excitation substantially longer than 1 millisecond to eiect said change from said rst to said second operating condition thereof, and an electrical energy-storage element included with resistors in a charge time-constant circuit and a discharge time-constant circuit in said control circuit, the time constant of said charge circuit being sub- 6 stantially equal to one millisecond and the time constant of said discharge circuit being in excess of two milliseconds thereby to prolong the amplitude of said control potential for a time inter- Val in excess of 2 milliseconds.

3. In a signal-seeking receiver, in combination, a tuner, means for causing said tuner rapidly to scan over its tuning range, a control tube having a cathode, an anode, and control electrode and having in its anode-cathode circuit a relay for controlling said means, means including a rst resistor for deriving a potential from incoming signals of predetermined strength and for impressing said potential between said cathode and controlelectrcde of said tube to change the position of said relay and thereby to terminate the scanning action effected by said tuner, the rapidity of said scanning causing said derived potential to vary through a cycle of amplitude change of duration normally too short to permit said change of position of said relay, and a condenser connected between said control electrode and said cathode and coupled through a second resistor to said potential deriving means, said condenser and said second resistor being included in a condenser charge circuit and having values selected to provide a time constant substantially equal to the duration of said cycle of amplitude change and said first, resistor being included with said condenser in a condenser discharge circuit having a time constant long in comparison to said charge time constant thereby to maintain the value of said impressed potential over a time interval long as compared to the interval during which said impressed potential has substantial amplitude in the absence of said condenser.

ROY A. GULL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,063,295 Braden Dec. 8, 1936 2,174,566 Case Oct. 3, 1939 2,262,218 Andrews Nov. 11, 1941 2,304,871 Andrews Dec. 15, 1942 2,434,293 Stearns Jan. 13, 1948 

